Video signals are a way of transferring visual information from one point to another. The transfer of video signals may be as remote as transferring a signal from a cable head end unit to a television set (TV) in a cable television system or as local as transferring video signals from one internal chip to another internal chip located within a single television set or set-top box.
Traditionally, video transfer has been performed using analog signals. Recently, digital signals have been employed to transfer video signals. The recent trend toward digital signal transmission is a result of the improved quality and falling costs of digital transmission related components.
The transition away from analog video transfer to digital video transfer has, however, been a slow process. While cable companies and other video broadcasters have begun to transmit video information using digital signals, a large number of television sets, monitors, videocassette recorders, and other viewing equipment continue to use analog signals or analog connection equipment. The continued use of analog signals and analog connection equipment requires, in many cases, a conversion from digital to analog prior to viewing of the video signal.
A large part of the conversion from a digital signal to an analog signal focuses on the color component of the video signal. Digital video information and analog video information are transmitted using different color spaces. Color spaces vary depending on the mathematical representation used to indicate colors such as, for example, the coordinate system used to mathematically represent a color. Typically, analog video systems use the red, green, blue (RGB) color space to represent color while digital video systems typically use the YCbCr color space. YCbCr is the color space originally defined by ITU-R BT.601, an international standard established by the International Telecommunication Union (ITU), now used for all digital component video formats.
The Y component of the YCbCr color space represents the luma or black and white component of the digital video signal. The Cb and Cr components of the YCbCr color space represent chroma difference signals of the digital video signal. Subtracting the luma from the red and blue components forms the color difference components.
Because different color spaces are used by analog and digital signals, a conversion from a digital video signal to an analog video signal necessarily entails color space conversion. The color space conversions are performed by integrated circuits or other internal circuitry of a digital television decoder such as a set-top box. The quality of the color space conversions may vary greatly depending on the quality of the circuitry being used.
Evaluating the quality of the color space conversion has traditionally been very subjective in nature. The subjective evaluations typically rated color space conversion circuitry based on the opinion of a viewer after visual inspection. The subjective evaluation of color space conversion circuitry often leads to systems that perform what may be less than ideal conversions from digital video to analog video.